ABSTRACT We will develop mitochondria-targeted antioxidants (MTAs) and imaging probes that will mitigate cardiotoxicity and enhance antitumor efficacies of chemotherapeutic drugs. We will use doxorubicin (DOX), a front-line antitumor agent in breast cancer treatment. DOX causes delayed dose-dependent cardiotoxicity. Clinically, this side effect is managed with conventional antioxidants and iron chelators. This proposal provides a new adjuvant approach in breast cancer chemotherapy. Its genesis is based upon the following discoveries: 1) MTAs (e.g., Mito-Q, a synthetic drug analog of an endogenous antioxidant, Co-enzyme-Q, present in the mitochondrial respiratory chain) inhibit DOX-mediated cardiotoxicity in a preclinical animal model and in cardiomyocytes, and 2) MTAs (Mito-Q and Mito-CP, a nitroxide targeted to mitochondria) cause antiproliferative and cytotoxic effects in breast cancer cells (MCF-7 and MDA-MB-231) but not in non- transformed breast epithelial cells (MCF-10A) and significantly enhance DOX-induced breast cancer cell toxicity. We hypothesize that mitochondria-targeted antioxidants enhance DOX-mediated antitumor effects but attenuate DOX cardiotoxicity. Response to chemotherapy will be monitored by using the mitochondria-targeted technetium-labeled imaging agents (99mTc-Mito10-MAG3) in a chemically-induced breast carcinoma animal model. Specifically, we will: (i) Investigate the cytotoxic effects of MTAs alone and with DOX in breast cancer cells, (ii) Assess the cytotoxic effects of MTAs and DOX in breast cancer cells overexpressing multi-drug resistant protein, (iii) Evaluate the adjuvant chemotherapeutic effects of MTAs and DOX in an in vivo breast cancer model, and (iv) Assess the cardioprotective and oxy-radical scavenging effects of MTAs in DOX- treated cardiomyocytes and in DOX-treated rat cardiomyopathy model. These aims will be accomplished using HPLC-fluorescence and HPLC-electrochemical detection techniques, scintimammography and echocardiography. Abnormal generation of reactive oxygen species will be detected using novel species- and target-specific probes. We will develop innovative MTA-based adjuvant therapy that can be used to inhibit DOX-induced cardiotoxicity. This research may potentially lead to novel ways for improving the therapeutic efficacy of DOX and other antitumor agents used in breast cancer treatment.